Capacitive sensors have a wide range of applications, and are among others used for the detection of the presence and/or the position of conductive body in the vicinity of an antenna electrode. As used herein, the term “capacitive sensor designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which may be identical with or different from transmitting antenna electrodes—at which the influence of an object or living being on the electric field is detected.
The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmitting electrode, building up an electric field to a receiving electrode, and the displacement current induced at the receiving electrode is measured. The measured displacement current depends on the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
The capacitive coupling strength is generally determined by applying an alternating voltage signal to an antenna electrode and by measuring the current flowing from that antenna electrode either towards ground (in the loading mode) or into a second antenna electrode (in coupling mode). This current may be measured by a transimpedance amplifier, which is connected to the sensing electrode and which converts the current flowing into the sensing electrode into a voltage proportional to the current.
When a capacitive sensor is used for occupancy detection, in a vehicle seat, the impedance between the antenna electrode and ground depends not only on the presence or absence of an occupant on the seat but also on other conducting bodies in the vicinity of the antenna electrode. One of these conducting bodies is the (typically metal) seat frame. Depending on the car model, the seat frame may or may not be electrically connected to the vehicle frame and this has an influence on the measured impedance between the antenna electrode and ground.
U.S. 2007/0192007 A1 disclose an occupant classification system comprising so-called “identifying” and “monitoring” circuit configured to identify whether the seat pan, the seat back frame and the seat heating element are grounded to a circuit ground.